Protein kinases (“PKs”) are enzymes that catalyze the phosphorylation of hydroxy groups on tyrosine, serine and threonine residues of proteins (Hunter, T., Cell 50 (1987) 823-829). The consequences of this seemingly simple activity are staggering; cell growth, differentiation and proliferation, i.e., virtually all aspects of cell life in one way or another depend on PK activity. Furthermore, abnormal PK activity has been related to a hose of disorders, ranging from relatively non-life threatening diseases such as psoriasis to extremely virulent diseases such as glioblastoma (brain cancer).
The PKs can be conveniently broken down into two classes, the protein tyrosine kinases (PTKs) and the serine-threonine kinases (STKs).
One of the prime aspects of PTK activity is their involvement with growth factor receptors. Growth factor receptors are cell surface proteins. When bound by a growth factor ligand, growth factor receptors are converted to an active form which interacts with proteins on the inner surface of a cell membrane. This leads to phosphorylation on tyrosine residues of the receptor and other proteins and to the formation inside the cell of complexes with a variety of cytoplasmic signaling molecules that, in turn, effect numerous cellular responses such as cell division (proliferation), cell differentiation, cell growth, expression of metabolic effects to the extracellular microenvironment, etc. For a more complete discussion, see Schlessinger, J., and Ullrich, A., Neuron, 9 (1992, 383-391, which is incorporated by reference, including any drawings, as if fully set forth herein.
Growth factor receptors with PTK activity are known as receptor tyrosine kinases (“RTKs”). They comprise a large family of transmembrane receptors with diverse biological activity. At present, at least nineteen (19) distinct subfamilies of RTKs have been identified. An example of these is the subfamily designated the “HER” RTKs, which include EGFR (epidermal growth factor receptor), HER2 (human epidermal growth factor receptor 2), HER3 and HER4. These RTKs consist of an extracellular glycosylated ligand binding domain, a transmembrane domain and an intracellular cytoplasmic catalytic domain that can phosphoiylate tyrosine residues on proteins.
Another RTK subfamily consists of insulin receptor (IR), insulin like growth factor I receptor (IGF-IR) and insulin receptor related receptor (IRR). IR and IGF-IR interact with insulin, IGF-I and IGF-II to form a heterotetramer of two entirely extracellular glycosylated α subunits and two β subunits which cross the cell membrane and which contain the tyrosine kinase domain.
Another RTK subfamily is referred to as the platelet derived growth factor receptor (“PDGFR”) group, which includes PDGFR alpha, PDGFR beta, colony-stimulating factor I receptor (CSF-IR), c-kit and flt 3. These receptors consist of glycosylated extracellular domains composed of 5 immunoglobin-like loops and an intracellular domain wherein the tyrosine kinase domain is interrupted by a kinase inert domain.
Another group which, because of its similarity to the PDGFR subfamily, is sometimes subsumed into the latter group is the fetal liver kinase (“Flk”) receptor subfamily. This group, containing extracellulos immunoglobulin loops made up of kinase insert domain-receptor fetal liver kinase-1 (KDR/Flk-1), and fins-like tyrosine kinase 1 (Flt-1 and Flt-4).
A further member of the tyrosine kinase growth factor receptor family is the fibroblast growth factor (“FGF”) receptor subgroup. This group consists of four receptors, FGFR1-4, and many ligands. Although there is considerable alternative splicing, generally the receptors consist of a glycosylated extracellular domain containing 3 immunoglobin-like loops and an intracellular domain in which the tyrosine kinase sequence is interrupted by regions of a kinase insert domain.
Still another member of the tyrosine kinase growth factor receptor family is MET, often referred to as c-Met also known as human hepatocyte growth factor receptor tyrosine kinase (hHGFR). c-Met is thought to play a role in primary tumor growth and metastasis.
A more complete listing of the known RTK subfamilies is described in Plowman et al., DN&P, 7 (1994) 334-339, which is incorporated by reference, including any drawings, as if fully set forth herein.
In addition to the RTKs, there also exists a family of entirely intracellular PTKs called “non-receptor tyrosine kinases” or “cytoplasmic tyrosine kinases.” This latter designation, abbreviated “CTK,” will be used herein. CTKs do not contain extracellular and transmembrane domains. At present, over 24 CTKs in 11 subfamilies (Src, Frk, Btk, Csk, Abl, Zap70, Fes, Fak, Jak, LIMK and Ack) have been identified. The Src subfamily appear so far to be the largest group of CTKs and includes Src, Yes, Fyn, Lyn, Lck, Blk, Hck, Fgr, and Yrk. A further important group of CTKs is the Abl family including Abl and Arg. For a more detailed discussion of CTKs, see Bolen, J. B., Oncogene, 8 (1993) 2025-2031, which is incorporated by reference, including any drawings, as if fully set forth herein.
The serine/threonine kinases, STKs, like the CTKs, are predominantly intracellular although there are a few receptor kinases of the STK type. STKs are the most common of the cytosolic kinases; i.e. kinases that perform their function in that part of the cytoplasm other than the cytoplasmic organelles and cytoskelton. The cytosol is the region within the cell where much of the cell's intermediary metabolic and biosynthetic activity occurs; e.g., it is in the cytosol that proteins are synthesized on ribosomes. The STKs include CDk2, Raf, the ZC family of kinases, the NEK family of kinases, and BUB1.
RTKs, CTKs and STKs have all been implicated in a host of pathogenic conditions including, significantly, cancer. Other pathogenic conditions which have been associated with PTKs include, without limitation, psoriasis, hepatic cirrhosis, diabetes, angiogenesis, fibrosis, restenosis, ocular diseases, rheumatoid arthritis and other inflammatory disorders, immunological disorders such as autoimmune disease, cardiovascular disease such as atherosclerosis and a variety of renal disorders.
With regard to cancer, two of the major hypotheses advanced to explain the excessive cellular proliferation that drives tumor development relate to functions known to be PK regulated. That is, it has been suggested that malignant cell growth results from a breakdown in the mechanisms that control cell division and/or differentiation. It has been shown that the protein products of a number of proto-oncogenes are involved in the signal transduction pathways that regulate cell growth and differentiation. These protein products of proto-oncogenes include the extracellular growth factors, transmembrane growth factor PTK receptors (RTKs), cytoplasmic PTKs (CTKs) and cytosolic STKs, discussed above.
In view of the apparent link between PK-related cellular activities and wide variety of human disorders, it is no surprise that a great deal of effort is being expended in an attempt to identify ways to modulate PK activity. Some of these have been made to identify small molecules which act as PK inhibitors. WO 2006/066913 relates to imidazolopyridinyl benzamide derivatives and related compounds as src and lck kinase inhibitors.
WO 2004/024897 relate to imidazopyridines as modulators for the IgE immune response in the treatment of allergic and proliferative diseases.